Polarization conversion system

ABSTRACT

A reflector, light source, phase retarder, and linear polarizer configured to increase a single-polarization output beam by converting wrong-polarization light into correct-polarization light. The linear polarizer reflects the wrong-polarization light and transmits the correct-polarization light. The phase retarder converts the wrong-polarization light into correct-polarization light, which is then transmitted by the linear polarizer.

This is a divisional of prior application Ser. No. 10/086,936, filedFeb. 28, 2002 now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to light projectors, and morespecifically to a projector light source with an improvedsingle-polarization output light beam.

2. Background Art

In many applications, it is advantageous to employ a light sourceproviding a light beam of a single polarization. For example, when usedwith single-polarization light, spatial light modulators (SLMs) offerimproved visual quality of their modulated light output. It has beencommon practice to obtain the single polarization by discarding light ofthe other polarization. This is undesirable for many reasons, such asreduced output intensity, increased power consumption, increased heatgeneration, and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood more fully from the detaileddescription given below and from the accompanying drawings ofembodiments of the invention which, however, should not be taken tolimit the invention to the specific embodiments described, but are forexplanation and understanding only.

FIGS. 1-5 show various embodiments of a polarization conversion systemaccording to this invention.

FIG. 6 shows one embodiment of a method of operation of a polarizationconversion system according to this invention.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of a polarization conversion system10, including a light source 12 such as an arc, a plasma lamp, or othersuitable means. In many applications, it will be desirable that thelight source generate a light beam having a small ètendue. The systemfurther includes a reflector 14 shaped and positioned relative to thelight source so to reflect the light beam into a substantiallycollaminated beam. The system further includes a quarter-wave phaseretarder 16 (“λ/4”) which rotates the polarization of the light passingthrough it by 45 degrees. The system also includes a linear polarizer 18which transmits light of a first polarization and reflects light ofanother polarization. One embodiment of a suitable linear polarizer isan optical substrate with a micro-structured wire grid type polarizersurface.

Operation of the system will be explained with regard to light that isreflected by the reflector. The reader will understand that the sameprinciples apply to light that travels directly from the light source tothe polarizer without first being reflected by the reflector. The lightemerges from the light source with a random polarization (L_(random)),is reflected by the reflector, passes through the quarter-wave phaseretarder, and encounters the linear polarizer. Some of the light(L_(parallel0)) happens to be of the correct polarization, and istransmitted through the linear polarizer to emerge as a first componentof the output beam.

Some of the light (L_(orthogonal)) is of the wrong polarization, and isreflected by the linear polarizer. It then passes back through thequarter-wave phase retarder, which rotates its polarization by 45degrees, is reflected by the reflector, and passes again through thequarter-wave phase retarder, where its polarization is rotated by anadditional 45 degrees. Now, when it encounters the linear polarizer, thelight is of the correct polarization (having made two passes through thequarter-wave phase retarder, for a total of a half phase of polarizationshift), and it emerges as a second component of the output beam(L_(parallel1)).

Unless ideal components can be utilized, there will be losses as thelight beams traverse the system. The better the components, the lowerthe losses, and the greater contribution the second component(L_(parallel1)) makes to the overall brightness of the output beam.

One suitable linear polarizer is the ProFlux polarizer available fromMoxtek, Inc, 452 W 1260 N, Orem, Utah 84057. Some details of the linearpolarizer are shown in U.S. Pat. Nos. 6,108,131, 6,122,103, and6,288,840. Additional information regarding such polarizers can be foundat http://www.moxtek.com and http://www.profluxpolarizer.com.

FIG. 2 shows a second embodiment 20 of a system utilizing thisinvention. It contains a light source 12, reflector 14, and linearpolarizer 18 as in the first embodiment. However, the phase retarder 26is layered on the reflector rather than being configured with thepolarizer. The phase retarder coating causes a quarter-wave retardationwhen passed through twice (passing into, then reflected back outthrough). In some embodiments, this coating may retard differently inthe two directions, and may retard differently depending upon the angleof incidence.

FIG. 3 illustrates a third embodiment of a polarization conversionsystem 30. It includes a light source 12, as well as a quarter-wavephase retarder 16 and a linear polarizer 18. The retarder and polarizercan, in this embodiment, be made approximately half the size they wouldbe in the other embodiments, because the reflector 32 is approximatelyone half the size of the reflectors in the embodiments of FIGS. 1 and 2.The system further includes a reflector 36. The reflector 36 issubstantially hemispherical, with a reflective concave surface.

Correct polarization light from the light source will be transmitted bythe linear polarizer as a first component (L_(parallel0)) of the outputbeam. Light of the wrong polarization will be reflected by thepolarizer, off the reflector 32, through the light source into theinterior chamber of the reflector 36 combination. Eventually, it willbounce back through the light source, off the reflector 32, make anotherpass through the retarder, and be of the correct polarization when itarrives at the polarizer, from which it will emerge as a secondcomponent (L_(parallel1)) of the output beam. In some embodiments, thelight will be re-emitted as randomly polarized light, rather than merelypassing through as indicated above. The embodiment of FIG. 3 will, ofcourse, generate a narrower output beam than those of FIG. 1 or 2,assuming the same general scale of components.

FIG. 4 shows a fourth embodiment 40 of a system utilizing thisinvention. It includes a radio-frequency (RF) driven plasma light source42, such as that available from Fusion, Inc. of 7524 Standish Place,Rockville, Md., USA. The light source includes a quartz bulb 44 to whichis affixed a quarter-wave phase retarder 16 and within which is asubstantially spherical light cavity. In some embodiments, the quartzbulb may have a flat spot to facilitate bonding of the phase retarder.The light source also includes a specular high reflectivity ceramiccoating 46 or other suitable reflector, which is advantageouslydeposited so as to encapsulate the bulb and the phase retarder. Thereflector includes a substantially non-reflective opening 48 which may,in some embodiments, be located substantially opposite the phaseretarder. The angles from the perimeter of the phase retarder to theedges of the opening define a field of vision, denoted in FIG. 4 bydotted lines. Ideally, the linear polarizer 18 should encompass thisfield of view. In some embodiments, the linear polarizer could belocated within the opening 48.

Some of the light created by the RF-induced plasma (not shown) willescape through the opening and contact the linear polarizer. A portionof that light that is of the correct polarization will be transmitted asa first component (L_(parallel0)) of the output beam. Thewrong-polarization portion will be reflected back into the plasma. Afterperhaps making some number of bounces off the reflective coating 46, thewrong polarization light may pass through the phase retarder, bereflected back through the phase retarder, pass through the opening 48,and be transmitted through the linear polarizer as a second component(L_(parallel1)) of the output beam.

FIG. 5 shows a fifth embodiment 50 of a system similar to that of FIG.4. The light source 52 has a reflective coating (not shown) on itsinterior surface, and a first 54 and a second 56 non-reflective windowthrough which the light beam can emerge. It includes a reflector 14,quarter-wave phase retarder 16, and linear polarizer 18, which are sizedappropriately. Light emerges from the first window and contacts thelinear polarizer. If it happens to be of the correct polarization, it istransmitted as a first component of the output beam. Otherwise, it isreflected back through the first window, emerges from the second window,passes through the quarter-wave phase retarder, is reflected by thereflector, makes a second pass back through the quarter-wave phaseretarder, passes through the second window, and emerges from the firstwindow. This time when it encounters the linear polarizer, it is of thecorrect polarization, having made two passes through the quarter-wavephase retarder, and becomes a second component of the output beam.

FIG. 6 illustrates one exemplary embodiment 60 of a method of operationof a polarization conversion system utilizing this invention. Light isgenerated (61) with plural polarizations, typically random polarization.It is conducted (62) to a polarizer. If (63) a component of the light isof the correct polarization, it is transmitted (64) as one component ofan output beam. If, however, a component of the light is of the wrongpolarization, it is reflected (65) for correction. In the process ofcorrection, the phase of the light's polarization is altered (66). Inembodiments, the phase-altered light is then reflected (67) back towardthe polarizer. In some embodiments, its phase may again be altered (68),if that is necessary in order for it to be of the right polarization. Inother embodiments, it may be possible to achieve the correctpolarization in a single operation, such as by passing it through ahalf-wave phase retarder. If the half-wave phase retarder is onlyoperative in a single direction, the phase-altered light may bereflected directly back through the (in this direction inert) phaseretarder on its way to the polarizer. If not, it will be necessary tobypass the phase retarder such as by reflecting around it by a series ofmirrors. Ultimately, the corrected-polarization light is transmitted(64) as a second component of the output beam. In other embodiments, itmay be desirable to use a phase retarder which rotates the light somefraction of a phase other than one quarter or one half. In those cases,the wrong-phase light may bounce between the polarizer and the reflectorseveral times before its polarization finally becomes correct and itemerges from the polarizer.

Reference in the specification to “an embodiment,” “one embodiment,”“some embodiments,” or “other embodiments” means that a particularfeature, structure, or characteristic described in connection with theembodiments is included in at least some embodiments, but notnecessarily all embodiments, of the invention. The various appearances“an embodiment,” “one embodiment,” or “some embodiments” are notnecessarily all referring to the same embodiments.

If the specification states a component, feature, structure, orcharacteristic “may”, “might”, or “could” be included, that particularcomponent, feature, structure, or characteristic is not required to beincluded. If the specification or claim refers to “a” or “an” element,that does not mean there is only one of the element. If thespecification or claims refer to “an additional” element, that does notpreclude there being more than one of the additional element.

Those skilled in the art having the benefit of this disclosure willappreciate that many other variations from the foregoing description anddrawings may be made within the scope of the present invention. Indeed,the invention is not limited to the details described above. Rather, itis the following claims including any amendments thereto that define thescope of the invention.

What is claimed is:
 1. An apparatus comprising: a reflector; a lightsource for producing light of a first polarization and light of a secondpolarization; a linear polarizer which substantially passes light of thefirst polarization and substantially reflects light of the secondpolarization; a phase retarder disposed between the reflector and thelinear polarizer; and wherein the light source is disposed between thephase retarder and the linear polarizer.
 2. The apparatus of claim 1wherein the phase retarder is mechanically coupled to the reflector andhas substantially a same shape as the reflector.
 3. The apparatus ofclaim 2 wherein the phase retarder and the reflector are substantiallyparabolic.
 4. The apparatus of claim 1 wherein the light sourcecomprises a substantially spherical light cavity having a reflectiveinner surface, and having a first window and a second window through thereflective inner surface, the first and second windows being disposed atsubstantially opposite sides of the light cavity.
 5. The apparatus ofclaim 4 wherein the phase retarder and the reflector are disposedsubstantially in an optical path with the first window, and the linearpolarizer is disposed substantially in an optical path with the secondwindow.
 6. The apparatus of claim 5 wherein the phase retarder, thereflector, and the linear polarizer are of substantially planar shape.7. The apparatus of claim 1 wherein the light source comprises: anRE-driven plasma light source having a bulb, and having a reflectivecoating surrounding the bulb and the phase retarder.
 8. A systemcomprising: a linear polarizing means for transmitting light of a firstpolarization and for reflecting light of a second polarization; phaseretarder means for altering polarization of the light of the secondpolarization reflected from the linear polarizer means; reflector meansfor reflecting the altered polarization light from the phase retardermeans back to the linear polarizer means.